Control rod drives

Nuclear Radiation Effects. Components of a nuclear reactor system that require lubrication include control-rod drives, coolant circulating pumps or compressors, motor-operated valves, and fuel handling devices, and, of course, are exposed to varying amounts of ionising (14). 

Table 4.1-5 shows in these records, there has been 3 control rod drive failures. Assume 100 plants in the U.S. with an average of 30 control rods/plant and 10.7 years of experience in this database. Estimate, the mode, 90% and 10% confidence limits for the failure rate. 

Adjustable speed motor driven feedwater pumps and high-capacity control rod drive pumps with backup power improve the safety margin by improving correct operator response to non-routine events. 

Excess reactivity Failure of automatic rod control Heat imbalance lemper- ature increase Control rod drive Operator action, interlocks HIE... 

This is a letter report from JBF Associates Inc., to Sandia National Laboratories (SNL) summarizing JBF s efforts to analyze dependent (common cause) failures and failures caused by harsh environments. The information used for the analysis was ta)cen from over 1000 failure reports (mostly abstracts of LERs that were assembled for other studies). The 26 groups of components selected for study are accumulators, batteries, cables, control rod drives,... 

In addition, multiple personal monitors are often used for situations in which a worker is exposed to a nonuniform radiation field, in an attempt to assess the region of the body receiving the highest deep dose equivalent. Approaches to the use of multiple personal monitors vary widely, and the number used and their locations depend on the particular work activity. For example, during work inside a steam generator, where the radiation fields are potentially isotropic, a total of 12 to 14 personal monitors may be placed at specific locations on both the front and the back of the body, and on top of the head. In other work situations, when the radiation field may be relatively directional but variable (e.g., during control-rod drive maintenance in a boiling-water reactor) the individual may wear all of the personal monitors at locations on the front of the body. 

Nuclear Boiler Assembly. This assembly consists of the equipment and instrumentation necessary to produce, contain, and control the steam required by the turbine-generator. The principal components of the nuclear boiler are (1) reactor vessel and internals—reactor pressure vessel, jet pumps for reactor water circulation, steam separators and dryers, and core support structure (2) reactor water recirculation system—pumps, valves, and piping used in providing and controlling core flow (3) main steam lines—main steam safety and relief valves, piping, and pipe supports from reactor pressure vessel up to and including the isolation valves outside of the primary containment barrier (4) control rod drive system—control rods, control rod drive mechanisms and hydraulic system for insertion and withdrawal of the control rods and (5) nuclear fuel and in-core instrumentation,... 

Reactor Assembly. This assembly (Fig. 3) consists of the reactor vessel, its internal components of the core, shroud, top guide assembly, core plate assembly, steam separator and dryer assemblies and jet pumps. Also included in the reactor assembly are the control rods, control rod drive housings and the control rod drives. 

Each fuel assembly that makes up the core rests on an orificed fuel support mounted on top of the control rod guide tubes. Each guide tube, with its fuel support piece, bears the weight of fuur assemblies and is supported by a control rod drive penetration nozzle in the bottom head of the reactor vessel. The core plate provides lateral guidance at the top of each control rod guide tube. The top guide provides lateral support for the top of each fuel assembly. 

Control rods occupy alternate spaces between fuel assemblies and may be withdrawn into the guide tubes below the core during plant operation. The rods are coupled to control rod drives mounted within housings welded to the bottom head of the reactor vessel. The bottom-entry drives do not interfere with refueling operations. A flanged joint is provided at the bottom of each housing for case of removal and maintenance of the rod dri ve assembly. 

Control Rod Drive System. Positive cure reactivity cuulrol is maintained by the use of movable control rods interspersed throughout the core. 

A cut-away schematic of a BWR equipped with external coolant pumps is shown in Fig. 17. The recirculation system comprises the external piping, pumps, and valves located at the lower region of the vessel. The penetrations through the bottom of the vessel that contain the control rod drive mechanisms that are vital for controlling the reactivity of the core are not shown in Fig. 17. Of particular importance, as far as the integrity of the reactor is concerned, is the control rod drive tubes and related mechanisms, because they are in contact with the coolant and... 

Fig. 17 Cut-away schematic of a BWR vessel and associated piping system for a BWR equipped with external coolant pumps. The underlined labels are indicating regions of IGSCC. The control rod drive mechanisms and penetrations that are located at the bottom of the vessel are not shown.

Nuclear Reactor with a Hole in the Head On March 6,2002, personnel repairing one of the five cracked control rod drive mechanism (CRDM) nozzles at Davis-Besse Nuclear Plant, Oak Harbor, Ohio, discovered extensive damage to the reactor vessel head. The reactor vessel head is a dome-shaped structure made from carbon steel housing the reactor core. The reactor vessel head is placed such that it can be removed when the reactor is shut down to allow spent nuclear fuel to be replaced with fresh fuel. The CRDM nozzles connect motors mounted on a platform above the reactor vessel head to control rods inside the reactor vessel. Reactor operators withdraw control rods from the reactor core to start the operation of the plant and insert the control rods to shut down the operation of the reactor. 

The control rod drives and the ion chamber hangers are under normal operating conditions. There are 27 control rod drives installed in the reactor. 

The specified lifetime of the control rod drives is set up to be 120000 hours. At present time a complex of the material tests in support of the extension of the lifetime is under way. 

FBTR operating experience has improved the confidence level in the design and operation of core, sodium systems, control rod drive mechanisms, fuel handling machines, steam water system and SG leak detection system. 

For the management of the Control Rod Drive Mechanism System for the reactors WER-1000 operated in Czech Republic and in Ukraine, a computer-based system has been developed in Skoda and is in operation since 1996 on the South - Ukraine NPP and then in Chmelnicka NPP and Temelin NPP. 

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